Skylights provide effective internal lighting for buildings, maximizing visual comfort and reducing energy usage from artificial lighting.
A skylight may include a rooftop element through which sunlight enters the skylight structure, the sunlight being transmitted through the skylight structure to the interior of the building. For example, a building skylight may also include a channel through roof trusses, the channel being disposed between the rooftop element and the interior opening of the skylight. Alternatively, a tubular skylight may include a rooftop element and a light conducting channel that is a tube, depending downwardly from the rooftop element and terminating at a room interior.
Sunlight received by the rooftop element varies in direction and incident angle during a single day. In early morning and late afternoon hours, the incident angle at which sunlight strikes the rooftop element of a skylight is relatively low. Furthermore, at sunrise and sunset, sunlight is attenuated due to its relatively longer passage through the Earth's atmosphere. Conversely, at mid-day, sunlight's incident angle upon a skylight rooftop element is relatively high. During the course of a day, the sun's path through the sky relative to a skylight rooftop element is arcuate relative to the horizon. Furthermore, at mid-day, the sunlight incident angle upon a skylight rooftop element at the Winter solstice is low, while the sunlight incident angle at the Summer solstice is high, with the incident angle varying throughout the year between those two extremes.
It has been found that the irradiance from sunlight arriving at a skylight from a low incident angle may be further reduced before reaching the interior of a building structure, as the sunlight at a low incident angle tends to be reflected several times within the skylight structure, and thereby lessened, before reaching the interior of the building. Furthermore, it has been found that the irradiance received within a building interior from sunlight arriving at a skylight from a high incident angle may be undesirably strong, causing for example “hot spots” within the building interior, inasmuch as such sunlight arrives at the building interior through the skylight structure with fewer reflections within the skylight structure, and thereby with greater retained brilliance.
In view of the foregoing, it would be advantageous to control the illuminance within a building received from a skylight throughout the day, and during the change of seasons, as the incident angle of sunlight changes.